Cathode ray tube



June 4, 1940. G. B. BANKS 2,203,483

CATHODE RAY TUBE Filed Jan. 15, 1937 lZig-i 06ft iCTIA/f (OILS INVENTORGEORGE BALDWIN BANKS ATTORNEY Patented June 4, 1940 PATENT OFFICEGATHODE RAY TUBE George Baldwin Banks, Chelmsford, England, assignor, toRadio Corporation of America, a corporation of Delaware ApplicationJanuary 15, 1937, SerialNo. 120,706 In Great Britain January 24, 1936 r10 Claims. (01. 250-453) This invention relates to cathode ray tubes andis applicable both to tubes of the oscillograph type, such as arecommonly employed for television reception and like purposes, and alsototubes of the type sometimes termed transmitter tubes such as arecommonly employed for television transmission purposes. In the formertype of tube thescreen electrode towards which the cathode ray is.projected is usually prepared with or carries a layer of fluorescentmaterial and in the latter type of tube (so-called iconoscope) thescreen electrode consists of a mosaic of minute photo-sensitive cells.

In cathode ray tubes of the types referred to,

it is commonly desired to apply a relatively high potential to thescreen electrode towards which the cathode ray is projected. Forexample, in the oscillograph type of tube with a fluorescent screen, itis common to constitute the fluorescent 1; screen by a layer offluorescent material upon a metal plate in order that highintensity ofillumination may be obtained without burning of the screen. Fortelevision receiver cathode ray tubes which are to be utilized forprojection purposes, this use of a metal plate as part of the screenstructure is, in practice, almost a necessity. Again, in the iconoscopetype of tube a metal back plate forms part of the screen. structure,this back plate cooperating with the tiny parconstitute a plurality ofminute condensers having a common electrode In this and in similar caseswhere the metal plate forms part of the screen it is common to tilt thescreen at an. angle of about 45" to the axis of the electron gun inorder to facilitate. the. projection of imageson to the mosaic screen,or to facilitate viewing of or projection of images built up on thefluorescent screen, as the case may be. t 40 There are a number ofserious objections to the use of a screen structure tilted at 45 to thegun axis. Referring to the drawing, Fig. 1 shows diagrammatically thepath travel. of a cathode ray beam for purposes of explaining theinvention while Fig. 2 shows schematically one embodiment of theinvention. One of these objections is as maybe seen from Figure 1 of theaccompanying drawing that the length of the path of the cathode ray fromgun to screen: is: not constant (compare lengths OA and OB for positionsA and Bof the ray) as the ray explores the screen and this involves thatsharp focusing of the cathode ray is not obtained over the whole screenarea, This ,obj ection is particularly serious jinz the fluorescentlscreen type of tube and comticles of photo-electric material in themosaic to pensation for such focusing error is difficult, especiallywhere magnetic focusing of the ray is employed. A second disadvantage isthat if the deflecting forces applied to produce scanning action of theray are such as would give a rectangular scanned area upon a screenperpendicular to the gun axis, the area actually scanned upon a tiltedscreen is not a rectangle but a trapezium, and although this type ofdistortion can be corrected for, the correction involves a certainamount of difiiculty and/or provisionof additional apparatus.

The present invention avoids the disadvantages and difficulties abovereferred to.

According to this invention a cathode ray tube comprises an electron gunand a screen structure which is mounted transversely withrespect to thegun axis and is also off-set from the gun axis and means are providedfor setting up between the gun and the screena ray deflecting fieldhaving lines. of force running parallel to the surface of the screen,the section of this field (viewed in the direction of the said lines offorce) being v such that it bends the cathode rays through the anglenecessary to cause it to fall upon the screen and in such manner thatthe path traced upon the screen as the result of the application ofpredetermineddeflecting force to the ray (other than the forces due tothe aforementioned field) is the same as would be traced upon a screenperpendicular to the gun axis were the same deflecting forces appliedbut the aforementioned field omitted. 9

Preferably the screen surface is parallel to the gun axis and preferablyalso the aforementioned field is a magnetic field of substantiallyuniform .drical portions l and 2 merged into one another,

the axis of the two cylinders being perpendicular. That. envelopeportion 2 which corresponds to the hammer head is considerably shorterthan the other and the said shorter cylindrical portion of theenvelopelies for its greater part to one side of the axis of the longercylindrical portion. In other words the hammer head portion is notsymmetrical to the hammer shaft portion the two portions joining nearone end of the hammer head portion. At that end of the hammer shaftportion remote from the ,hammer head? portionis carried the usual electron gun system not shown but indicated, as to approximate position bythe reference 0, the axis of the gun corresponding to the axis of thecylindrical envelope portion l in which it is mounted. The screen 4,which in, the case of a perpendicularly to the axis or the hammen headportion of the envelope near that end of said portion which is furthestfrom the axis of the other envelope portion, That end wall of the hammerhead portion of the envelope which is furthest from and parallel to thescreen is preferably constituted by a separate circular plate 5 of goodoptical qualities to facilitate the projection of light on to the screen(where the said screen is a mosaic) or the projectionof light from thescreen (where the said screen is fluorescent) In use, ray deflectingforces, shown in the drawing as conventional magnetic deflecting coils,well known in the art, it being understood that static deflection couldbe used optionally, are appliedto thecathode ray, as in the usual way,insuch manner that a rectangular area would be traced upon a screenperpendicular to the gun axis and means are provided for bending thecathode ray round the corner between the two cylindrical envelopeportions in such manner that a similarly shaped rectangular area istraced upon the actual screen, which is, as stated, parallel to and notat right angles to the gun axis. The said means are constituted by anelectromagnetic field of triangular section this field being generated,by an electromagnet having triangular pole pieces'only one of which,namely 6, is shown by broken lines. mer head portion of the envelope ispositioned between these triangular pole pieces so that theelectromagnetic lines of force run through the hammer head portion ofthe envelope in a direction parallel to the surface of the screen (per--pendicular to the plane of the paper in Fig. 2). The section of thetriangular field is that of a right angled isosceles triangle and therelative position of the tube and of the pole pieces is such that thehypotenuse of the triangle runs at 45 to boththe mutually perpendicularaxes of the mutually perpendicular portions of the tube envelope. Itwillthus be seenthat the triangular magnetic field acts in arnanner some:-what analogous to that of a prism of optics. The

cathode ray in positions (0A, QB, 00) entering] this field are bentround more or less in circular manner until they proceed towards thescreen. 1

A ray projected from the gun in a direction DA on theside of the gunaxis 00 remote from the screen'will be bent round by-the triangularfield, and will, upon leaving said field, proceed in a direction towardsthe edge A of the screen which is nearest the gun. Similarly a rayprojected from the gun'in a direction OB on the side oi'the gun axistowards the screen will be bent round by the field in such manner as to'reach thescreen adjacent that edge Bffwhich is furthest from the gun.By correctly positioning the tube with relation to the pole pieces,rtheresult may be achieved, that the shape of the scanned area on the screenis the same as that which would be obtained were there no triangularfield and the screen were perpendicular to the gun axis. In practice, anexternal lens system 8 may be placed with its axis, corresponding to.

The hamdeposition 9 is provided in the hammer head portion of the tubeenvelope at least over the. cylindrical walls thereof, and means may beprov vided for applying a desired fixed, potential to this coating whichmay, for example, be constituted by a silver deposit. Such a coatingwill serve to ensure a'uniiorm electric field within the correspondingpart of the tube.

Having described my invention, what I claim is:

l. A cathode ray tube comprising in combination, an envelope of twoapproximately cylindrical portions having their axes mutuallyperpendicular, an electron gun at the, end of one said portion remotefrom the other said portion the axis of the said gun corresponding tothe axis of one said portion, a screen at'the end of the othersaid-portion remote'from the one said portion mounted perpendicularly tothe axis of the other said portion, an; optical end w'all mounted at theend of the other said portion parallelto the screen and means forestablishing an external magneticfield of substantially uniformintensity and of triangularlsection, the

tions remote from theother said portion, the

axis of'said gun corresponding to the axis of one said portion,a screenat the other end of the other said portion remote from the one saidportion mounted perpendicular to the other said portion, an optical endwall mounted at the end of the other said portion parallel to thescreen, and means for establishing an external magnetic field ofsubstantially uniform intensity and of triangular section, thehypotenuse of said triangle section being placed at an angle of 45degrees" to the axis of both portions. 7

3. An electronic device comprising 'afirst cylindrical envelope,anpptical end wall at one end of said envelope, 9. target electrode atthe other end of saidenvelope, a second'cylindrical envelope joinedto'fthe first cylindrical envelope intermediate the optical end walland, target, an 3 electron gun structure positioned within and at theunjoined end of the second envelope and beyond theconfines of said firstcylindrical envelope the axis of said electron gun coinciding with theaxis of said second envelope, both of said axes being perpendicular tothe axis of the first cylindrical envelope, and means for establishingan electromagnetic field of constant intensity intermediate the targetand end wall, the direction of said field being perpendicular to theaxis of both of said cylindrical envelopes.

4. An electronic device comprising a first cylindricalenvelope-anoptical end wall at one end of magnetic field of constant intensity andtriangular cross-section intermediate the target and end diate theoptical end wall and target, an electron gun structure positioned withinand at the unwall, the direction of said field being perpendicular tothe axis of both of said cylindrical'en velopes.

5. An electronic device comprising a first cylindrical envelope, anoptical end Wall at one end of said envelope, a target electrode at theother end of said envelope, a second cylindrical envelope joined to thefirst cylindrical envelope intermejoined end of the second envelope andbeyond the confines of said first cylindrical envelope, the axis of saidelectron gun coinciding with the axis of said second envelope, both ofsaid axes being ofsaid envelope, a target electrode at the other end ofsaid envelope, a second cylindrical envelope joined to the firstcylindrical envelope intermediate the optical end wall and target, anelectron gun structure positioned within and at the unjoined end of thesecond envelope and beyond the confines of said first cylindricalenvelope, the

axis of said electron gun coinciding with the axis of said secondenvelope, both of said axes being perpendicular to the axis of the firstcylindrical envelope, means for establishing an electromagnetic field ofconstant intensity intermediate the target and end WalLthe direction ofsaid field being perpendicular to the axis of both of said cylindricalenvelopes, and acylindrical focusing electrode positioned onthe innersurface of each perpendicular to the axis of the first cylindricalenvelope, means for establishing an electromagnetic field of constantintensity and triangular cross-section intermediate the target and endwall, the direction of said field being perpendicular to the axis ofboth of said cylindrical envelopes, and a cylindrical focusing electrodepositioned on the inner surface of each of the cylindrical envelopes.

8. An electronic device comprising a first cylindrical envelope, anoptical end Wall at one end of said envelope, a targetelectrode at theother end of said envelope, a second cylindrical envelope joined to thefirst cylindrical envelope intermediate the optical end wall and target,an electron gun structure positioned within and at the unjoined end ofthe second envelope and beyond the confines of said first cylindricalenvelope, the axis of said electron gun coinciding with'the axis of saidsecond envelope, both of said axes being perpendicular to the axis ofthe first cylindrical envelope and beyond the confines of said firstcylindrical envelope, means for establishing an electromagnetic field ofconstant intensity and isosceles cross-section intermediate the targetand end wall, the direction of said field being perpendicular to theaxis of both of said cylindrical envelopes, and a cylindrical focusingelectrode positioned on the inner surface each of the cylindricalenvelopes.

9. An electronic device comprising a first cylindrical envelope, anoptical envelope at one end of said envelope, a target electrode at theother end of said envelope, a second cylindrical envelope joined to thefirst cylindrical envelope intermediate the end wall and target, theaxis of said second cylindrical envelope being perpendicular to the axisof the first cylindrical envelope, means to direct a beam of electronsalong the axis of said second cylindrical electrode, said means beinglocated at'the unjoined end of the second cylindrical envelope, meansfor setting up an electromagnetic field of constant intensity Whosedirection is perpendicular to the axis of both of said cylindricalenvelopes for projecting the directed beam of electrons onto the targetelectrode, and focusing cylindrical electrodes positioned on the innerwall of said cylindrical envelope.

10. An electronic device comprising a first cylindrical envelope, anoptical envelope at one end of said envelope, a target electrode at theother end of said envelope, a second cylindrical envelope joined to thefirst cylindrical envelope intermediate the end wall and target, theaxis of said second cylindrical envelope being perpendicular to the axisof the first cylindrical envelope, means to direct a beam of electronsalong the axis of said second cylindrical electrode, said means beinglocated at the unjoined end of the second cylindrical envelope, meansfor setting up an electromagnetic field of constant intensity andtriangular cross-section whose direction is perpendicular to the axis ofboth of said cylindrical

